Radio transmitter



{March 1942- c. s. COCKERELL ET AL 2,277,955

RADIO TRANSMITTER Filed April 26, 1941 INVENTORS CHRISTOPHER s. COCKERELL BfEOPGE P. PAR/(ER m AT'i'ORNEY Patented Mar. 31, 1942 RADIO TRANSMITTER Christopher Sydney Cockerell, Danbury, and

George Percy Parker, Chelmsford, England, assignors to Radio Corporation of America, a cor- Doration of Delaware Application April 26, 1941, Serial No. 390,484 In Great Britain March 27, 1940 4 Claims.

The present invention relates to radio transmitters and particularly to radio transmitters for aircraft. It is not, however, so limited.

The principal object of the invention is to enable the maximum voltage, over the frequency range of a radio transmitter, to be maintained without requiring the services of a skilled operator.

In certain radio transmitters, particularly in certain radio transmitters for aircraft, the maximum power that can be applied to the aerial is limited by the maximum voltage that the various aerial insulators will withstand. This maximum voltage will be obtained for a given aerial from a given amount of aerialpower at the minimum tuning frequency, where the aerial impedance is at a maximum. If the amount of power that can be applied to the aerial be limited to the power which will give the maximum voltage at the minimum frequency, it follows that at higher frequencies the aerial will be operating at voltages which are much less than the permissible maximum voltage. To obtain a constant maximum voltage over a tuning range will require that the power delivered to the aerial shall be increased as the frequency is increased, an operation which would normally require the services of a skilled operator.

According to the invention, a radio transmitter having a final amplifier stage includes in the anode circuit of said stage a current indicator means for indicating the optimum anode supply and means, gang controlled with the tuning reactance, for shunting a variable portion of the anode current around said indicator means, whereby said indicator means provides a substantially fixed indication of the optimum anode supply irrespective of the frequency to which the transmitter is tuned.

The means for varying the grading of said current indicator may comprise a variable resistance connected across the current indicator and the resistance may be gang controlled with a Variable aerial tuning inductance.

An embodiment of the invention is illustrated in the sole figure of the accompanying drawing wherein PT indicates the final amplifier valve of the transmitter, the anode of the valve being connected to a source S of high positive potential through a high frequency choke HFC in series with a resistance R, milliammeter MA, and a rheostat RH. Output is taken from valve PT through a blocking condenser C and applied to the aerial inductance AL, the point of application being, if desired, adjustable as indicated in coni ventional manner. The aerial A is connected to a point on the aerial inductance and the lower end of the inductance is earthed.

Shunted across the milliammeter and resistance R is a variable resistance PR, the tapping point p of which may be adjusted in conventional manner. The amount of aerial tuning inductance can be selected by a connector 10''. This connector is adapted to short circuit a requisite number of turns of the aerial tuning inductance for the desired frequency at which the transmitter is to operate. The tap p and connector p" are gang controlled as indicated at G.

On the scale of the milliammeter a loading mark is provided and the operator is required to adjust the anode feed to this mark.

As the frequency of operation is reduced (or increased) more (or less) aerial tuning inductance will be required and the adjustment of the aerial tuning inductance p" will, owing to the gang control, be accompanied by an increase (or decrease) in the amount of resistance shunting the milliammeter Thus, the more inductance that is included in the aerial circuit, the higher will be the value of the resistance shunting the milliammeter, and consequently, since the feed will be adjusted to a given mark on the scale of the instrument, the true anode feed will be reduced.

The value of the milliammeter shunt resistance is so chosen that the aerial voltage will remain approximately constant over the frequency range of the transmitter, while the value of the current shown on the anode feed milliammeter remains constant.

In the figure the maximum aerial tuning inductance is represented as included, and consequently the maximum amount of resistance is indicated as shunted across the milliammeter.

We claim:

1. In a radio transmitter circuit arrangement, a tuning device for the output circuit of a discharge tube in a power output stage, an ammeter and a resistor in said output circuit, a rheostat in shunt with said ammeter and resistor, means for adjusting the anode current in said tube to an optimum value for any desired adjustment of said tuning device, and means providing gang control of said tuning device and said rheostat, said rheostat being operable to produce a substantially fixed ammeter reading for all adjustments of said tuning device.

2. The combination according to claim 1 and including an inductance coil in said tuning device, said coil having a connection at one end to an adjustable tap for short-circuiting a desired number of convolutions of said coil, said tap being adjustable by said gang control means.

3. A transmitting amplifier system comprising a final stage including the anode circuit of a discharge tube, means including a current-responsive instrument in said anode circuit for indicating the optimum anode current, a tuning reactance coupled to said anode circuit, and gangcontrolled means for simultaneously adjusting the sensitivity of said instrument and the tuning of said reactance, whereby the indication on said instrument of optimum anode current for different operating frequencies of said reactance is maintained substantially constant.

4. In the art of adjusting the final amplifier stage of a radio transmitter, where said stage ineludes the anode circuit of a discharge tube, a resistor and an ammeter in the anode circuit, a potentiometer in shunt with said resistor and armneter, and a tuning reactance coupled to said anode circuit, said potentiometer and tuning reactance being gang controlled, the method of adjusting the anode current to an optimum value for a given frequency to which said reactance is tuned which comprises varying the amount of anode current bypassed through said potentiometer as a function of the frequency to which said reactance is tuned, and causing said ammeter to make substantially the same indication for all adjustments of said tuning reactance.

CHRISTOPHER SYDNEY COCKERELL. GEORGE PERCY PARKER. 

